An economic collapse on the scale of the 2008 financial crisis begins (perhaps triggered by China or the technology sector). At this point in the 2008 election cycle, the unemployment rate was 4.9%, and by November 2008 it had risen to almost 7%. Imagine the impact that kind of economic debacle would have on a Democratic presidential candidate’s prospects.

Clinton (age 68) or Sanders (age 74) – whichever is the Democratic nominee – dies (from natural causes, a plane crash, or whatever) or is incapacitated (e.g., by a severe stroke), and the Democratic Party finds it difficult to rally around a new and strong candidate. The GOP candidate would likely benefit from the resulting chaos.

Michael R. Bloomberg (or someone else) runs as an independent, and the election results in no presidential candidate winning a majority of votes in the the Electoral College. The presidential election would then be decided by the House of Representatives. (The voting process is by state delegation, and you can find the details here.) The bottom line is that the GOP would probably decide on the next President, and they would likely vote for the GOP candidate – Trump.